There are significant differences between the two main types of screen cylinders with slotted apertures that are used for screening cellulosic fibrous material pulp in the pulp and paper industry, namely milled screen cylinders, and screen cylinders fabricated from discrete elements, such as bars or wires. The screen cylinders formed from discrete elements, including wedge wire screen cylinders, are perceived to have higher capacity than milled screen cylinders because there is more potential open area. However conventional wedge wire screens also have significantly lower debris removal efficiency than conventional milled cylinders. For example in one test conducted between a wedge wire cylinder and a milled cylinder which had similar configurations and were manufactured by the same company, the milled cylinder had a debris removal efficiency of over 77% while the wedge wire cylinder had a debris removal efficiency of about 40%, using the same pulp furnish. In said co-pending application various techniques and procedures are illustrated and described for enhancing the functionality of wedge wire cylinders so that they more closely approximate the debris removal efficiency of the wedge wire screen cylinders compared to milled screen cylinders, including by avoiding the Coanda effect, and by providing a plug phenomena during the negative pulse cycle. However it has also been recognized as desirable for many years to increase the capacity of milled cylinders so that they more closely approximate that of cylinders made of discrete elements (such as wedge wire cylinders).
According to the present invention screen cylinders, and methods of utilization and manufacture thereof, are provided which have greatly enhanced utility compared to conventional milled and discrete element screen cylinders. For example according to the present invention it has been found that even compared to commercial screen cylinders with popular contours (such as those sold by CAE ScreenPlates and known as the “D-PROFILE”™) may greatly increase capacity while maintaining at least as good debris removal efficiency [capacity enhancement is not worthwhile if it results in significant debris removal efficiency loss] by making seemingly very minor changes in the configuration of the grooves at the slots. As a matter of fact milled cylinders can be so improved by practicing the present invention that a milled cylinder with smaller slots can have a higher capacity than a conventional wedge wire cylinder with larger slots, something considered impossible in the prior art. However the invention is not limited to improvements in milled cylinders, but also can significantly enhance the performance of wedge wire, or other discrete element, cylinders, i.e. the debris removal efficiency of discrete element screen cylinders can be improved dramatically.
The effect on capacity with the present invention increases as slot sizes get smaller and decreases as slots get larger. The invention is not expected to have any significant effect at slot sizes above 1 mm, but the invention has a dramatic effect for slot sizes below 1 mm, and especially between about 0.05–0.5 mm (and all narrower ranges within that broad range). The general goal in screening is increasingly to use the smallest possible slots for the highest possible debris removal efficiency. The constraining factor is loss of capacity and other operational problems if slots are too small. The present invention will allow further decreases in slot sizes than was possible before.
Exactly what theory explains the highly advantageous results that can be achieved according to the present invention is not presently well understood. With respect to discrete element cylinders, it is believed that the elimination of the Coanda effect, compared with a funneling type action adjacent the slot, are responsible, but exactly how the funneling action adjacent the slot creates the favorable flow conditions that achieve the desired results according to the present invention is not presently well understood.
Something that may explain the advantageous results according to the present invention is the ability of the invention to deal with fiber flocs. While those in the art have a tendency to consider that the fiber stock being screened has homogeneously distributed fibers, such as illustrated two dimensionally in FIG. 8 (the real fibers and flocs are three dimensional), in fact typically flocs of fibers are formed in the stock slurry, as illustrated in FIG. 9. To put the floc formation into a practical perspective, if a certain type of fiber will form flocs at one percent consistency within 100 micro-seconds, at three percent consistency only 10 micro-seconds may be needed to form a floc. It is believed that the particular contour of the cylinders according to the present invention de-flocculates the clusters into individual fibers separated by a film of water. If the fibers are not de-flocculated (fluidized), especially smaller slots and sharp edges or slot entrances with radiuses that are too small, will make it difficult for “intact” flocs to enter the slot. Then the flocs will be rejected as debris while the water portion between the flocs passes into the accepts, causing thickening of the rejects, and rejecting good fibers. It is believed that with the construction according to the invention it is considerably easier to guide flocs toward the narrow slot openings where they gradually become compressed in the smaller slot section. The fiction force created between the compressed fiber flocs and the side walls inside the slots can now be overcome, as the slot entrance remains “open” instead of being plugged with flocs of fibers. With an unplugged or “open” slot entrance it is then possible for the pressure drop over the screen cylinder, combined with small positive pulses from the rotation of the rotor foil or protrusion leading edges, to effectively push the compressed flocs out from the narrow slots into the relief grooves on the accepts side.
Another potential advantage of the construction according to the invention is the enhanced debris removal efficiency. With small slots and long fibers, conventional modern screening operations using, for example, OCC secondary fibers have to compromise between efficiency and fractionation (rejecting) the valuable long fibers. In order not to fractionate out the valuable long fibers it is necessary to operate the screens with fairly high passing velocities in the slots, which creates a high push/pull type force on the fibers (or flocs) in the slots, keeping them from moving into the accepts. However in order to have good efficiency and debris removal it is generally accepted that the passing velocity in the slots has to be fairly low as higher passing velocities have a detrimental affect on efficiencies. However the constructions according to the invention allow the screen cylinders to be operated with lower passing velocities in the slots, without increased fractionation of long fibers.
In an exemplary embodiment of the invention, a screen cylinder having a screening surface and an accepts surface on opposite faces thereof is provided for screening pulp flowing in a flow direction to separate accepts from rejects. The screen cylinder includes a plurality of grooves defined in the screening surface generally transverse to the flow direction; a slot defined in each of at least a plurality of the grooves; and each of the grooves having an upstream curved or substantially planar surface, and a curved or substantially planar downstream surface having a first portion thereof remote from the slot defining an angle that is about 5–40 degrees, preferably 10–30 degrees, with respect to the flow direction, and a second portion thereof adjacent the slot defining an angle that is about 45–80 degrees with respect to the flow direction. The cylinder may be made from a plate with the grooves milled and the slots cut therein, or alternatively, may be made from a plurality of bars or wires mounted so that they are substantially parallel to each other, defining the grooves and slots therebetween.
In the bars or wires context, substantially each of the bars or wires preferably includes a transition between the upstream and downstream surface thereof, wherein the transition includes a portion substantially parallel to the flow direction and a substantially sharp edge between the upstream surface and the transition. Moreover, substantially each bar or wire preferably includes a transition between the upstream and downstream surface thereof contoured so that turbulence is formed at the slot and so as to substantially avoid the Coanda effect at the slot.
The downstream surface is preferably substantially convex. Also, there may be a clearly defined break in the downstream surface between the first and second portions thereof, and the slots preferably have a substantially uniform width of between about 0.05 mm–0.5 mm.
In another exemplary embodiment of the invention, a screen cylinder having a screening surface and an accepts surface on opposite faces thereof is provided for screening pulp flowing in a flow direction to separate accepts from rejects. The screen cylinder includes a plurality of grooves defined in the screening surface generally transverse to the flow direction; a slot defined in each of at least a plurality of the grooves; and the screen cylinder grooves and slots being contoured so that the slots have a nozzle or discharge coefficient C at least 10% greater at substantially the same debris removal efficiency compared to a wedge wire screen cylinder having the same slot width, for the same pulp furnish, wherein grooves of the wedge wire screen cylinder have a sloped downstream surface, an upstream surface making an angle of about 70–110 degrees with respect to the flow direction, and a rounded transition between the upstream and downstream surfaces. The screen cylinder may have a coefficient C that is about 20–50% greater compared to the wedge wire screen cylinder. In addition, the screen cylinder preferably has a coefficient C equal or proportional to greater than about 0.5 when the velocity of the flow of pulp through the slots is between about 1.5–5.0 m/s, and the pulp has a consistency between about 0.8–5%. The screen cylinder may have slots about 0.17 mm wide, or an equivalent, when screening TMP has greater capacity and at least about 30% less shives in the accepts compared to the wedge wire screen cylinder which has 0.15 mm slots, or an equivalent. In another arrangement, the screen cylinder may have slots about 0.15 mm wide, or an equivalent, has substantially the same operating characteristics at typical average passing velocities of 1.5–2 m/s in the slots as the wedge wire screen cylinder which has 0.2 mm slots, or an equivalent, when screening CTMP at a consistency of about 1.5%, or the equivalent.
In yet another exemplary embodiment of the invention, a screen cylinder having a screening surface and an accepts surface on opposite faces thereof is provided for screening pulp flowing in a flow direction to separate accepts from rejects. The screen cylinder includes a plurality of grooves defined in the screening surface generally transverse to the flow direction and each including an upstream surface and a downstream surface: a slot defined in each of at least a plurality of the grooves; and the screen cylinder grooves and slots being contoured so that the cylinder includes a transition between the upstream and downstream surfaces thereof contoured so that turbulence is formed at the slot and so as to substantially avoid the Coanda effect at the downstream/upstream transition area, and define a funnel at the slot improving at least one of capacity or debris removal efficiency while not adversely affecting the other of capacity or debris removal efficiency, and substantially de-flocculating the pulp.